Photosensitive insulating material



July 28, 1970 c. WOOD E'TAL PHQTOSENSITIVE INSULATING MATERIAL FiledNov. 30, 1965 FIG. 2

INVENTORS CHARLES wooo G.SANJ|V KAMATH BY JAMES H NEYHART ATTORNEY-SUnited States Patent 9 PHOTOSENSITIVE INSULATING MATERIAL Charles Wood,Pittsford, G. Sanjiv Kamath, Rochester,

and James H. Neyhart, Penfield, N.Y., assignors to Xerox Corporation,Rochester, N.Y., a corporation of New York Filed Nov. 30, 1965, Ser. No.510,636 Int. Cl. G03g 5/08 US. Cl. 96-15 4 Claims ABSTRACT OF THEDISCLOSURE A xerographic plate comprising a supporting substrate havingon one surface thereof a photoconductive insulating layer, saidsubstrate having an electrical resistance of less than saidphotoconductive layer, with said photoconductive layer comprising atleast one inorganic-photoconductor compound of the Group IIIV elementsdispersed throughout a resinous binder, with said photoconductivematerial having a resistivity of at least ohm-cm., and saidphotoconductive layer being capable of supporting an electrostaticcharge in the dark, and disssipating a portion of said charge inresponse to impinging electromagnetic radiation.

This invention relates to the art of imaging, and more specifically, toan improved xerographic system.

In the xerographic process as described in US. Pat. 2,297,691, a baseplate of relatively low electrical resistance such as metal, paper,etc., having a photoconductive insulating surface thereon iselectrostatically charged in the dark. The charged coating is thenexposed to a light image. The charges leak off rapidly to the base platein proportion to the intensity of the light to which any given area isexposed. After such exposure the coating is contacted with electrostaticmarking particles in the dark. These particles adhere to the areas wherethe electrostatic charges remain forming a powder image corresponding tothe electrostatic image. The powder image can then be transferred to asheet of transfer material resulting in a positive or negative print, asthe case may be, having excellent detail and quality. Alternatively,'where the base plate is relatively inexpensive, as of paper, it may bedesirable to fix the powder image directly to the plate itself.

It has been previously known that certain inorganic photoconductors in abinder system have utility for xerographic purposes. For example, it isknown to use an inorganic photoconductive pigment in anon-photoconductive resin binder for xerographic purposes and it isfurther known to use the same inorganic pigments in a photoconductivebinder. Furthermore, while it has been known that compounds of the GroupIII-V members of the Periodic Table also possess limited photoconductiveproperties, due to certain inherent characteristics, the use of thesematerials in electrophotography has been generally avoided.

While basically some of the above-mentioned inorganic materials havebeen found useful under certain circumstances in electrophotographicprocesses, it has been found that there are inherent disadvantages totheir use. One disadvantage, for example, is that spectral response overmost of the light visible region cannot generally be obtained withoutresorting to additional treatments of the photoconductors. A seconddisadvantage to the use of the above-mentioned materials for xerographicplates is that any additional treatment of the photoconductive materialsis substantially limited by the agents which may be used effectively togive the desired results. That is, if it is nec- "ice essary to increasethe spectral response of these photoconductive materials, such asbytreatment with a doping agent, this can be effectively accomplishedonly with a very restricted number of dopants. As a result of therelatively high starting impurity content and substantial stoichiometricinstability of the useful prior art photosensitive compounds, the numberof dopants or activating agents that may be utilized to affect a changein the electrophotographic properties of these compounds issubstantially limited. Furthermore, due to these inherent properties, itis extremely difficult to control and predict the results which will beobtained when treating with the effective activating agents. A furtherdisadvantage is that the above-mentioned instability significantlyaffects the electrical and optical properties of the photoconductivematerial. Still a further disadvantage of the above-mentionedphotoconductive materials is that they are limited by the type of chargeto which they may be exposed when used in a xerographic mode.

It is, therefore, an object of this invention to provide a xerographicplate which will overcome the above-noted disadvantages.

It is a further object of this invention to provide a process of using anovel xerographic plate.

Another object of this invention is to provide a novel xerographic platewherein spectral response can be attained over most of the light visibleregion without resorting to further treatment of the photoconductivernaterial.

Still a further object of this invention is to provide a novelxerographic plate wherein subsequent treatment of the photoconductivematerials is not limited by the agents which may be used effectively togive the desired results.

Yet, still a further object of this invention is to provide a novelxerographic plate wherein the stability of the plate is not dependentupon the photoconductive material utilized to make the plate.

An additional object of this invention is to provide a method ofpreparing a novel xerographic plate wherein the materials used to makethe plate are not limited by the steps of the process, such as the typeof charging required.

The foregoing objects and others are accomplished in accordance withthis invention, generally speaking, by providing a xerographic plateprepared by intimately mixing and grinding together a photoconductiveinsulating material in a high electrical resistance binder. Thephotoconductive materials of this invention, gallium phosphide, galliumarsenide phosphide, aluminum phosphide, boron phosphide, and mixturesthereof, prepared by the wellknown vapor transport process are doped ata temperature of about 1,000 0., preferably with oxygen or copper, priorto mixing with the resinous binder solution, in order to raise theresistivity of the material to at least 10 ohms-cm. The oxygen andcopper dopants are preferred in order to achieve optimum results. Thepreferred blend of the photoconductive insulating material with thebinder solution is about 12 parts of the photoconductive material perabout 5-1 part binder solution, by volume. It has been found that theseproportions produce a plate of superior xerographic properties,specifically with regards to plate sensitivity and spectral response. Ithas been found that when the photoconductive insulating materials ofthis invention are treated with an activator or dopant in such a mannerso as to raise their resistivities to at least 10 ohms-cm, and thenblended with the appropriate high electrical resistance binder thatthese compounds are useful for xerographic purposes. The resultingmixture is suitable as the photoconductive insulating layer of thexerographic plate and may be coated on any suitable support materialoffering a relatively lower electrical resistance than the coating suchas metal, paper, or suitable plastics as more fully described hereafter.The coating can be electrostatically charged and imaged in accordancewith the conventional xerographic imaging process as described in US.Pat. 2,297,691.

It is generally considered that the inorganic photoconductors known tohave utility for xerographic purposes such as those disclosed in US.Pats. 3,121,006 and 3,121,007 are limited in their application inasmuchas the spectral response of these materials can not ordinarily beattained over most of the light visible regions without resorting toadditional treatments of the photoconductive materials. Thephotoconductive materials of the present invention are suitable for usewhen it is desirable to utilize light sources covering most of the lightvisible regions without resorting to further treatment of thephotoconductive material, therefore demonstrating panchromaticproperties. Furthermore, although it is considered possible to extendthe spectral response of the inorganic photo conductors presently founduseful for xerographic purposes by additional treatments with activatingagents, it has been found that these photoconductive materials can onlybe treated effectively by a limited number of such agents. As a resultof the relatively high initial impurity content and substantialstoichiometric instability of the useful prior art photoconductivecompounds, the number of dopants or activating agents may be utilized toaffect a change in the electrophotographic properties of these compoundsis substantially limited. Furthermore, due to these inherent properties,it is extremely difficult to control and predict the results which willbe obtained when treating with the effective activating agents. It hasfurther been found that added treatments of the photoconductiveinsulating materials of this invention are not so limited. It ispossible to dope the Group IIIV compounds with a greater variety ofdopants as a result of the low impurity content and stoichiometricstability of these compounds. If it is found that difficulty developswhen utilizing one specific dopant or activating agent then it ispossible to readily substitute another equally effective butnon-detrimental dopant. It has still further been found that thephotoconductive insulating materials used in the course of thisinvention are much more controllable compounds than thosephotoconductive materials previously used. That is, they are notaffected by stoichiometric deviations when prepared and, therefore, donot suffer from electrical and optical deviations which is usually thecase when employing the inorganic photoconductive materials previouslyfound useful xerographically due to their wellknown unstablecharacteristics. It has also been determined that the photoconductiveinsulating materials of this invention can be prepared having eitherp-type or n-type conductivity properties which are more fully describedin US. Pat. 3,041,166. A material is referred to as being of the p-typewhen the majority charged carriers are holes and n-type when themajority charged carriers are electrons. Furthermore, the amphotericproperties of the photoconductive insulating materials of this inventionlend flexibility to the system such as in the type of charging required.

In accordance with this invention, it has been found that a xerographicplate can be prepared by intimately mixing together high resistancephotoconductive insulating material of the Group IIIV compounds With ahigh electrical resistance binder. More specifically, when gal liumphosphide, gallium arsenide phosphide, aluminum phosphide, boronphosphide and mixtures of these compounds are treated in such a mannerso as to effect a resistivity increase to at least ohms-cm. it has beenfound that these materials when combined with suitable binder materialsare quite useful for xerographic purposes. The binder material which isemployed in cooperation with the photoactive compounds is a materialwhich is an insulator to the extent that an electrostatic charge placedon the layer is not conducted by the binder at a rate to prevent theformation and retention of an electrostatic latent image thereon.Furthermore, the binders should not react chemically with thephotoactive compound. The binder material adheres tightly to the basematerial and provides an efficient dispersing medium for the photoactiveparticles.

The high resistivity photoconductive insulating material of thisinvention is prepared by controlled doping of the photoactive materialat a temperature of at least about 800 C. until the resistivity of thephotoconductive insulating material is raised to at least 10 ohm-cm. sothat when combined with the binding material the uniform photoconductiveinsulating layer will support an electrostatic charge in the dark. Theupper temperature limit of the doping step is limited only by therequirements of the system. The preferred range of the resistivity ofthe photoconductor is between about IO -10 ohm-cm. to produce optimumresults. Any other suitable means may be used to raise the resistivityof these compounds to the required critical value such as preferentialremoval of impurities from the photoconductive layer. However, thedoping technique is preferred inasmuch as it is less critical, forexample, than the purification process.

The doped photoconductive insulating material is reduced to a powderedform by grinding together in a ball mill or other suitable means untilthe size of the particles are sufficiently small so that they will notdestroy the insulating effect of the binder material mixed therewith.For maximum efficiency, it is preferred that the particle size of thephotoconductive material be at least 200 mesh or smaller. The finelydivided particles are then blended in the presence of a suitable solventwith a binder resin and mixed thoroughly until a viscous paste-liketexture is obtained. It is only essential that enough solvent be presentduring milling to give good grinding viscosity. The paste is thenapplied to the surface of a base plate in a thin uniform layer by anysuitable means such as with a brush, draw blade, by dipping, or byroller coating. At the end of the milling period, additional solvent maybe added and stirred into the mixture sufiicient to render it sprayable.The resulting composition can then readily be sprayed at roomtemperature onto a clean base plate.

The base substrate bearing the photoconductive insulated layer is thendried at a temperature sufficient to cause complete evaporation of thesolvent from the binder composition without destroying the stability ofthe binder resin. After the solvent has evaporated from the composition,the coating can be electrostatically charged and used in theelectrophotographic or xerographic process. Although the spectralresponse obtained when using the photoconductive materials of theinvention can be varied depending upon the desired results, it has beenfound that the preferred spectral range was determined to be in thevisible spectrum, that is, from about 4,000 to 7,000 A.

Thickness of the photoconductive insulating layer of the instantinvention is not critical and may vary from about 1 micron to over 200microns. When used, for example, in the process of electroradiographydescribed in US. Pat. 2,666,144, the photoconductive layer may besubstantially thicker than 200 microns. However, in the present system,it is preferred that the photoconductive layers be from about 20 to 115microns thick in order to obtain the maximum efiiciency of theelectrophotographic plate.

In general, the ratio between binder and the inorganic photoconductiveinsulating compound is from about 1 part binder and 10 partsphotoconductive to about 2 parts binder and 1 part photoconductor byvolume. The preferred blend is from about 1-2 parts photoconductor toabout 5-1 parts binder to produce a xerographic plate of maximumefficiency. The actual proportion will, of course, depend upon thespecific binder as well as on the properties and characteristicsdesired. As a general guide,

photoconductor to the surface of the backing member and which will forma smooth and useful surface for the ultimage deposition thereon felectrostatically charged powder particles.

Any suitable binder material may be used in the course of thisinvention. Typical such binder materials are those disclosed in U.S.Pats. 3,121,006 and 3,121,007. While the nature of the binder materialis not critical it does have a definite effect upon the lightsensitivity of the composite layer. In general, those binders havingstrongly polar groups such as carboxyl groups, chloride, etc. arepreferred over the straight hydrocarbon binders. It is believed thatinjection of carriers from the photoconductor to the binder isfacilitated through the presence of such groupings and further that thebonding of the photoactive compounds to the binder is improved thereby.Examples of such binder resins are polymerized butyl methacrylates,polyvinyl chloride, polyvinyl acetate, polyacrylic acid esters, andvinyl chloride-vinyl acetate copolymers. In addition, other suitablebinder materials are chargetransfer type photoconductive materials suchas disclosed in applications, Ser. Nos. 426,409; 426,423; 426,428;426,431 and 426,396, filed in the US. Patent Office on Jan. 18, 1965.

Any suitable solvent may be used in the course of this invention. Thesolvent used should be such as to readily evaporate and be asubstantially pure, organic, low boiling-point hydrocarbon solvent andshould not introduce impurities which would lower electrical resistanceof the coating. Typical such solvents are toluene, ethylene glycolmonoethyl ether acetate, xylene, benzene, methyl isobutyl ketone, ormixtures thereof. Preferred solvents used in the instant invention arebenzene, xylene, toluene, and methyl isobutyl ketone inasmuch as theyhave been found to give the most satisfactory results.

Any material suitable to raise the resistivity of the photoconductiveinsulating material of the invention to at least about ohm-cm. may beused in the course of this invention. Typical such doping materials arecopper, silver, iron, cobalt, gold, manganese, chromium, nickel, oxygen,and mixtures thereof. Generally, the oxygen and copper doping agents arepreferred inasmuch as the desired resistivity of the photoconductiveinsulating material is more readily obtained.

Any suitable backing material for the xerographic plate may be used inthe course of this invention. Generally, the preferred backing materialshould have an electrical resistance less than the photoconductive layerso that it will act as a ground when the film is electrostaticallycharged. Typical such materials are aluminum, brass, glass, aluminumcoated glass, steel, nickel, bronze, copper, engravers copper, engraverszinc, grained lithographic zinc, and paper. Other materials havingelectrical resistances similar to the aforementioned can also be used asbacking material to receive the photoconductive layer thereon. Othernonconductive materials such as thermoplastics may be used as thebacking for the xerographic plate. When used, however, it is necessaryto charge both sides of the xerographic plate according to the processset out in US. Pat. 2,922,883.

The invention is illustrated in the accompanying drawings in which:

FIG. 1 is a side sectional view of an exemplary xerographic processingapparatus employing the improved plate of this invention;

FIG. 2 is a side view of the improved xerographic plate of thisinvention.

An exemplary xerographic copying apparatus adapted to emp oy thexerographic plate of this invention in the form of a cylindrical drum isshown in FIG. 1. The drum, when in operation, is generally rotated at auniform velocity in the direction indicated by the arrow in FIG. 1 soafter portions of the drum periphery pass the charging unit 18 and havebeen uniformly charged, they come beneath a projector 19 or other meansfor exposing the charged plate to the image to be reproduced. Subsequentto charging and exposure, sections of the drum surface move past thedeveloping unit, generally designated 21. This developing unit is of thecascade type which includes an outer container or cover 22 with a troughat the bottom containing a supply of developing material 23. Thedeveloping material is picked up from the bottom of the container anddumped or cascaded over the drum surface by a number of buckets 24 on anendless driven conveyor belt 26. This development technique, which ismore fully described in US. Pats. 2,618,552 and 2,618,551, utilizes atwo element development mixture including finely divided, coloredmarking particles or toner and larger carrier beads. The carrier beadsserve both to deagglomerate the fine toner particles for easier feedingand charge them by virtue of the relative positions of the toner andcarrier material in the triboelectric series. The carrier beads withtoner particles clinging to them are cascaded over the drum surface. Theelectrostatic field from the charge pattern on the drum pulls tonerparticles off the carrier beads serving to develop the image. Thecarrier beads, along with any toner particles not used to develop theimage, then fall back into the bottom of container 22 and the developedimage moves around until it comes into contact with a copy web 27 whichis passed up against the drum surface by two idle rollers 28 so that theweb moves at the same speed as is the periphery of the drum. The tonerin the developing mixture is periodically replenished from a tonerdispenser not shown. A transfer unit 29 is placed behind the web andspaced slightly from it between rollers 28. This unit is similar innature to the plate charging mechanism 18 in that both operate on thecorona discharge principle. Both the charging device 18 and the transferunit 29 are connected to a source of high D.C. potential of the samepolarity identified as 31 and 32, respectively, and including a coronadischarge wire 33 and 34, respectively, surrounded by conductive metalshield. In the case of charging unit 18, voltage source 31 ispreselected to be of such a magnitude that it would produce a coronadischarge on the drum under almost any conditions of relative humidityand atmospheric pressure normally encountered which would tend to chargea conventional xerographic plate well above the desired voltage. Thisexcessively high potential source is preset and need not be adjustedbecause the retained voltage on the plate is controlled by theelectrical characteristics of the plate itself in such a way that anyexcessive current which flows through the plate during the coronadischarge is drained away by the voltage regulating characteristics ofthe plate. In the case of the corona discharge transfer unit, charge isdeposited on the back of web 27 and this charge is of the same polarityas the charge initially deposited on the drum and also opposite inpolarity to the toner particles utilized in developing the drum. Adischarge deposit on the back of web 27 pulls the toner particles awayfrom the drum by overcoming the force of attraction between theparticles and the charge on the drum. It should be noted at this pointthat many other transfer techniques can be utilized in conjunction withthe invention. For example, a roller connected to a high potentialsource opposite in polarity to the toner particles may be placedimmediately behind the copy web or the copy web itself may be adhesiveto the toner particles. After transfer of the toner image to web 27, theweb moves beneath a fixing unit 36 which serves to fuse or permanentlyfix the toner image to web 27. In this case, a resistance heating-typefixer is illustrated. However, here again, other techniques known in theart may also be utilized including the subjection of the toner image toa solvent vapor or spraying of the toner image with an adhesivefilm-forming overcoating. After fixing, the web is rewound on a coil 37for later use. After passing the transfer station, the drum continuesaround and moves beneath the cleaning brush 38 which prepares it for anew cycle of operation. It should be noted that this apparatus may alsobe operated at varying speeds by setting the corona discharge unit at ahigh enough voltage so that the plate will be charged fully at thehighest speed. Then, overcharging will not occur at the lower speedsbecause of self regulation by the plate.

Although the invention has been described in connection with coronacharging, it is to be understood that this is exemplary only, and thatthe self regulating plate may, in fact, be employed with any suitablecharging technique. Other difiicult charging methods include frictioncharging and induction charging as described in US. Pats. 2,934,649 and2,833, 930 and roller charging as described in US. Pat. 2,934,650.

FIG. 2 illustrates a xerographic plate 10 comprising backing material 11and a photoconductive insulating layer 12 comprising a binder material13 and inorganic photoconductive material 14. The selection of thesupporting substrate layer 11 is based upon the desired use of thexerographic plate, such as to give the plate additional strength or toprovide added flexibility in situations requiring it.

To further define the invention the following examples are intended toillustrate and not limit the particulars of the present system. Partsand percentages are by weight unless otherwise indicated. The examplesalso illustrate various preferred embodiments of the present invention.

EXAMPLE I Gallium phosphide crystals prepared by the well known vaportransport process are doped in an atmosphere of hydrogen carrier gas andwater vapor until the resistivity of the crystals is raised to at least10 ohm-cm. The reaction zone is maintained at a temperature of about1,000 C. and at a pressure of about 1 atmosphere. A controlled amount ofoxygen dopant, approximately less than 100 p.p.m., is introduced by wayof the carrier gas-water system. The resulting gallium phosphidecrystals are then ground in a ball mill until the crystals are reducedto about 300 mesh powder. A binder mixture comprising Lucite, apolymerized butyl methacrylate available from E. I. du Pont de Nemours &Co., in xylene, about 11-20 percent by weight, is prepared and blendedwith the gallium phosphide powder, 1 gram of powder per 1 ml. of bindersolution. The binder composition is thoroughly mixed with the galliumphosphide powder to a viscous paste. The paste is then coated onto analuminum conductive substrate in a thin uniform layer about 100 micronsthick. The resulting coated substrate is then heated to a temperature ofabout 150 C. in order to dry the photoconductive layer and expedite theevaporation of the solvent present. The resulting electrophotographicplate is charged to about 350400 volts by means of a laboratory Corotronunit powered by a high voltage power supply. The charging current is 0.1of a milliamp at 7,500 volts. A transparent positive USAF test chart isplaced on the charged gallium phosphide plate and exposed with a 75 wattphotofiood lamp. An exposure of about 100 footcandle seconds is requiredfor the gallium phosphide plate. The electrostatic latent image producedis then developed with electrostatic marking particles or toner.

EXAMPLE II The procedure of Example I is repeated excepting dopedgallium arsenide phosphide crystals of a resistivity of at least 10ohm-cm. are substituted for the doped gallium phosphide crystals. Theresulting xerographic plate has a slightly higher decay rate in the darkas compared to the gallium phosphide plate.

EXAMPLE III The procedure of Example I is repeated excepting a dopedmixture of gallium phosphide and gallium arsenide phosphide crystals ofa resistivity of at least 10 ohm-cm. is substituted for the dopedgallium phosphide crystals. The results obtained are similar to those ofExample II.

EXAMPLE IV The procedure of Example I is repeated excepting a dopedmixture of gallium phosphide and aluminum phosphide crystals of aresistivity of at least 10 ohm-cm. is substituted for the doped galliumphosphide crystals. The resulting xerographic plate has a dark decayrate slightly less than the plate of Example I.

Although the present examples were very specific in the terms ofconditions and materials used, any of the above listed typical materialsmay be substituted when suitable in the above examples with similarresults.

In addition to the steps used to prepare the xerographic plate of thepresent invention, other steps or modifications may be used ifdesirable. In addition, other materials may be incorporated in thexerographic plate of this invention which will enhance, synergize, orotherwise desirably effect the properties of materials presently used.For example, the spectral sensitivity of plates prepared in accordancewith the instant invention may be modified through the inclusion ofphotosensitizing dyes therein.

Anyone skilled in the art will have other modifications occur to himbased on the teaching of the present invention. These modifications areintended to be encompassed within the scope of this invention.

What is claimed is:

1. A xerographic plate comprising a supporting substrate having on onesurface thereof a photoconductive insulating layer, said substratehaving an electrical resistance less than said photoconductive layer,said photoconductive layer comprising a resin binder and an inorganicphotoconductor composition selected from the group consisting of galliumphosphide, gallium arsenide phosphide, aluminum phosphide, boronphosphide, and mixtures thereof, with said composition having aresistivity of at least 10 ohm-cm.

2. The plate of claim 1 in which the resistivity of the compound hasbeen increased by doping with an activator material selected from thegroup consisting of oxygen and copper.

3. A method of imaging which comprises applying an electrostatic chargeto a photoconductive layer comprising a finely-divided inorganicphotoconductor dispersed in a highly insulating resin binder, saidinorganic photoconductor being selected from the group consisting ofgallium phosphide, gallium arsenide phosphide, aluminum phosphide, boronphosphide, and mixtures thereof, said inorganic photoconductor having aresistivity of at least 10 ohm-cm, and exposing said charged layer to apattern of activating electromagnetic radiation to form a latentelectrostatic image on the surface of said photoconductive layer.

4. The method of claim 3 in which the latent electrostatic image isdeveloped with electroscopic marking material.

References Cited UNITED STATES PATENTS 3,043,958 7/1962 Diemer 25250l X3,121,006 2/1964 Middleton et al. 96-1.5 3,261,080 7/1966 Grimmeiss etal. 252-50"1 X GEORGE F. LESMES, Primary Examiner C. E. VAN HORN,Assistant Examiner U. S. Cl. X.R.

